title: C-arm angle measurement with accelerometer for brachytherapy: an accuracy study.
abstract: OBJECTIVE
  X-ray fluoroscopy guidance is frequently used in medical interventions. Image-guided interventional procedures that employ localization for registration require accurate information about the C-arm's rotation angle that provides the data externally in real time. Optical, electromagnetic, and image-based pose tracking systems have limited convenience and accuracy. An alternative method to recover C-arm orientation was developed using an accelerometer as tilt sensor.
  
  
  METHODS
  The fluoroscopic C-arm's orientation was estimated using a tri-axial acceleration sensor mounted on the X-ray detector as a tilt sensor. When the C-arm is stationary, the measured acceleration direction corresponds to the gravitational force direction. The accelerometer was calibrated with respect to the C-arm's rotation along its two axes, using a high-accuracy optical tracker as a reference. The scaling and offset error of the sensor was compensated using polynomial fitting. The system was evaluated on a GE OEC 9800 C-arm. Results obtained by accelerometer, built-in sensor, and image-based tracking were compared, using optical tracking as ground truth data.
  
  
  RESULTS
  The accelerometer-based orientation measurement error for primary angle rotation was -0.1 ± 0.0° and for secondary angle rotation it was 0.1 ± 0.0°. The built-in sensor orientation measurement error for primary angle rotation was -0.1 ± 0.2°, and for secondary angle rotation it was 0.1 ± 0.2°. The image-based orientation measurement error for primary angle rotation was -0.1 ± 1.3°, and for secondary angle rotation it was -1.3 ± 0.3°.
  
  
  CONCLUSIONS
  The accelerometer provided better results than the built-in sensor and image-based tracking. The accelerometer sensor is small, inexpensive, covers the full rotation range of the C-arm, does not require line of sight, and can be easily installed to any mobile X-ray machine. Therefore, accelerometer tilt sensing is a very promising applicant for orientation angle tracking of C-arm fluoroscopes.
authors.affiliation: Technische Universität München, Munich, Germany, thomas.wolff@mytum.de.
authors.forename: Thomas
authors.initials: T
authors.lastname: Wolff
authors.forename: Andras
authors.initials: A
authors.lastname: Lasso
authors.forename: Markus
authors.initials: M
authors.lastname: Eblenkamp
authors.forename: Erich
authors.initials: E
authors.lastname: Wintermantel
authors.forename: Gabor
authors.initials: G
authors.lastname: Fichtinger
mesh_terms: D061725:Accelerometry / Q000295:instrumentation*; D000465:Algorithms; D001918:Brachytherapy / Q000379:methods*; D002138:Calibration; D004867:Equipment Design; D005471:Fluoroscopy / Q000379:methods*; D006801:Humans; D011856:Radiographic Image Enhancement / Q000379:methods*; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results